1. Field of the Invention
The invention concerns a process for producing niobium Josephson junctions comprising a niobium base electrode, a niobium-oxide tunnel barrier and a subsequently deposited niobium counter electrode.
2. Description of the Prior Art
Josephson devices comprising one or more Josephson junctions are well known in the art. In the data processing field their application has been suggested particularly for memory cells but also for logic circuitry of a wide variety of layouts and functions.
Various materials have been investigated for both the junction electrodes and the junction barrier layers. All-niobium junctions, to which the present invention is related, exhibit some important advantages over, e.g.,lead-lead junctions. Niobium (Nb) is, from a technology point of view, a non-critical material with excellent mechanical stability. It also causes fewer diffusion problems and, desirably, its transition temperature T.sub.c is high.
In the past, all-niobium junctions have suffered from poor quality of the oxide layer leading to increased single particle currents or excess currents in the voltage range below the gap voltage. This resulted in a large spread in Josephson current density and in a low value of V.sub.m that determines the operational characteristics of the junction. The main problem has been that depositing a Nb counter electrode on a Nb tunnel barrier caused a degrading of the oxide barrier. Oxygen from the barrier reacted with the niobium resulting in the formation of lower oxides such as NbO and NbO.sub.2 thus causing a lower quality junction. V.sub.m values of only 10 to 13 mV could be achieved which are not sufficient for high quality Josephson devices required in today's applications.
Various attempts have been made to overcome these problems. In IBM Technical Disclosure Bulletin, Vol. 15, No. 11, April 1973, pp. 3316/17, the article "Multilayer Josephson Memory Device" discloses a sandwich structure employing intermediate Al layers to prevent degrading of the barrier layer when depositing the Nb counter electrode. The Nb/Al/Al.sub.2 O.sub.3 /Al/Nb structure avoids the problem of reactive material coming in contact with the oxide barrier. The Al layers become superconducting by their proximity to the Nb when kept sufficiently thin. The use of these proximity layers, however, results in a negative effect on the junction characteristics.
In IBM Technical Disclosure Bulletin, Vol. 21, No. 4, September 1978, p. 1652, the article "Quality Improvement of Josephson Junctions by Surface Treatments in the Barrier Layer" proposes to submit the barrier layer to an RF plasma treatment in a nitrogen atmosphere before depositing the top electrode. The improvement is caused by the replacement of absorbed oxygen molecules by nitrogen and by some additional interaction of nitrogen with the surface of the barrier. This allows the Nb counter electrode to react with a nitride or with nitrogen instead of oxygen. Tests have, however, shown that this process does not sufficiently prevent the Nb of the counter electrode from reacting with the oxygen of the barrier layer.
In IBM Technical Disclosure Bulletin, Vol. 22, No. 4, September 1979, pp. 1661/62, the article "All-Niobium Josephson Tunnel Junction with Improved Characteristic" proposes to provide a thin niobium carbide (NbC) layer on the barrier oxide prior to deposition of the Nb counter electrode to prevent the reaction between the Nb and the barrier layer oxide to take place. Again, this method has not been successful since it has proven to be too difficult to produce the required carbide layer without damaging the barrier layer.
All these approaches were quite different from the novel process herein described and did not result in the required improvement.
In IBM TDB, Vol. 17, No. 8, January 1975, p. 2446, the article "Making Josephson Tunnel Junctions" discloses a lead (Pb) junction structure in which the barrier layer is covered with a continuous film of gold to protect the barrier surface from contamination during subsequent lithographic steps. The gold diffuses into the low melting Pb. Although the use of the gold layer suggests some similarities with the present invention, the process conditions and parameters, the type of junctions to which the process is applied as well as the purpose and effect achieved are totally different: in the present process, applied to all-Nb junctions and not to Pb junctions, a non-continuous layer of e.g. gold is required in order to permit a subsequent partial oxidation of the barrier layer and to prevent adhesion problems that occur when the Nb counter electrode is deposited onto a continuous gold layer. These requirements and problems do not exist for the structure and materials of the prior art reference.